If there are some classes of “private” digital communications the federal government will not decrypt and read, what are they?
Since 9/11/01, the federal government has vastly expanded its interception of digital communications within the US. Public debate about this has mostly occurred after the fact in court cases and in Congressional hearings about the Patriot Act and other bills. As I recall, the justification for additional governmental invasions of privacy was that we needed to do this to protect our country from physical attack by foreign terrorists. However, the new legal authority and actual practice seem to have gone much beyond that, according to this LAT article.
Immigration and Customs Enforcement plans to hire 1,000 computer experts to fight cybercrime, including money laundering, arms trafficking, child pornography, and sex tourism. ICE also has password breaking facilities to "overcome encrypted defenses used by online criminals." It appears that the ICE Cyber Crimes Center investigates any crime as to which their technical capabilities are useful, without regard to whether there is an obvious international element. I'll bet a nice bottle of wine they're not getting judicial warrants for much of this. In any event, ICE is having jurisdictional disputes with other federal agencies with similar capabilities, and who knows if they are formally constrained by any cross-border requirement.
Some officials have returned to the private sector, citing frustration with fighting among the Homeland Security Department and other law enforcement agencies over leadership of cyber-security efforts.
We seem to be on a slippery slope. We've been persuaded that government must have extraordinary powers once thought denied it by the Bill of Rights in order to protect the nation from massive explosions and other violent terrorist attacks projected onto the homeland from abroad. With that decision behind us, we naturally begin to notice that there are conventional non-terrorist threats to our domestic safety that are more real, more prevalent, and actually result in hundreds or thousands of deaths every year, for example gang violence, drunk or reckless driving, and violations of food safety regulations. Why should government not use its enhanced powers to crack down on those criminal threats to our personal safety also? Indeed, why should it not do so for tax evasion, securities fraud, and property crimes also, or for any crime—even a "minor" one—for which the techniques are useful?
Back in the 20th Century we thought that government could not, without probable cause and judicial warrants, intrude into communications as to which the participants had a reasonable expectation of privacy. But if we know Big Brother can and does intercept any communication, it's not reasonable ever to have an expectation of privacy, is it? If we have not already reached a point where the government might intercept any of our digital communications (and crack our passwords and encrypted text if necessary), what types of communications are still reliably private? My guess is none.